V.
OPTICAL AND INFRARED SPECTROSCOPY
Academic and Research Staff
Prof. C. H. Perry
Dr. R. P. Lowndes
Graduate Students
J. F. Parrish
N. E. Tornberg
Jeanne H. Fertel
D. J. Muehlner
A.
IMPURITY-INDUCED
ABSORPTION IN PEROVSKITE FLUORIDES
The fundamental lattice vibrations and multiphonon bands in various perovskite fluand Young, 1 ',2 and the study of the far infrared
orides have been investigated by Perry
-1
) of these materials was initiated to find antiferro-1
confirmed a similar
magnetic resonance (AFMR) in KNiF 3 . A strong band at 49 cm
3
line reported by Richards which he assigned to AFMR. The same sample of KNiF 3 was
absorption spectra (below 100 cm-
subjected to magnetic fields of up to 150 kG at the Francis Bitter National Magnet Laboratory, M. I. T. A field of this magnitude would be expected to shift an AFMR frequency
-1
-1
or in
. No shifts were observed, however, in the strong band at 49 cm
by ~15 cm
several sharp weaker bands that were also observed (Fig. V-l). KNiF 3 has nearestneighbor spins oppositely directed along a cube axis, and is
because it undergoes no measurable distortion below TN (TN=
that the observed bands in KNiF
second sample of KNiF 3 ,
3
particularly interesting
2 7 50
).
Further evidence
were not in fact of magnetic origin was found when a
grown at a later time,
-1
displayed a considerably different
was greatly reduced in strength, and the weaker-1
bands were not evident (Figs. V-2 and V-3). The temperature dependence of the 49 cm
band was found to be opposite to that expected of AFMR. The dependence of the absorpspectrum.
The strong band at 49 cm
tion spectrum on the particular sample, and the lack of sensitivity to a strong magnetic
field indicate that the observed absorption bands are due only to impurities. Table V-l
compares some of the properties of various perovskite fluorides.
Examination of these
-1
, most
crystals at low temperatures has shown many absorption bands below 100 cm
KMgF 3 , a diamagnetic crystal, shows a band
of which are probably impurity-induced.
-1
-1
and there are also
which seems to correspond to the KNiF 3 band at 49 cm
at 52 cm
-1
. The spectrum again depends on the sample,
bands located at 64, 73, 88 and 135 cm
although not as strongly as is
system K(Mg/Ni)F
3
observed with KNiF
was also investigated,
3
(Fig. V-4).
The mixed crystal
and the frequency of the strongest band,
This work was supported principally by the Joint Services Electronics Programs
(U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E),
and in part by the U. S. Air Force (ESD Contract AF 19(628)-6066), and the M. I. T. Sloan
Fund for Basic Research.
QPR No. 89
KNiF
60
c50
40
Liq
He
Liq
N2
2 30
S20
10
20
30
40
60
50
WAVE NUMBERS
Fig. V-i.
7(
80C
90
100
cm
Transmission of KNiF
3
(sample #1)
at liquid-
helium and liquid-nitrogen temperature.
KNiF
70
#2
(24mm
60
LIQ. He
50
LIQ
KN
N2
F3
(2.4 mm)
LIO. He
40
-
SAMPLE
#1
SAMPLE
# 2
30
20
30
40
50
60
WAVE
80
70
NUMBERS
90
100
30
crr
Fig. V-2.
Transmission of KNiF
3
89
50
60
70
80 WAVE NUMBERS cm
90
Fig. V-3.
(sample
#2)
at
liquid-helium and liquid-nitrogen temperature.
QPR No.
40
KNiF
3
transmission spectra
at liquid-
helium temperature, showing comparison
between the two samples grown at different times.
(V.
Table V-i.
Crystal
KMgF
3
KZnF
3
KCoF
3
RbMnF
Properties of cubic perovskite fluorides.
Neel
Temperature
(-K)
Cubic Lattice Constant
at 300 'K (A)
diamagnetic
4. 000
3
KNiF
OPTICAL AND INFRARED SPECTROSCOPY)
Temperature at which
structure changes
(OK)
remains cubic
4. 014
275
4. 055
diamagnetic
114
4. 068
114 (tetragonal)
82
4. 240
cubic through 20
3
-1
varied smoothly with the relative concentrations of Mg and Ni.
cm -1,
-50
could
bands
be traced
through the whole
range
No other
with confidence
of concentration
(Fig. V-5).
Three other crystals which were investigated were K(Zn:l%Ni)F
RbMnF
3,
3
and
70
z
o
70
# 557
r
70
Fig. V-4.
TI
<
1i
1
o
50
KCoF
3
2
spectra at liquid-
transmission
helium temperature. The three samples
from different sources show almost identical spectra.
(I.9mm)
KMg F
0
KMgF
i
110-
90
70
WAVE NUMBERS (cm
130
150
1)
In
(Figs. V-6 and V-7).
each of these crystals absorption bands were found
which may be attributable to impurities.
the lowest frequency lattice band).
bands,
(The strongest band shown for RbMnF 3 is
KCoF 3 exhibits a large number of absorption
of varying intensities and widths.
an AFMR band,
It is possible that one of these may be
as no field measurements have been made.
-l
It is interesting that the strongest impurity bands found (KNiF 3:49 cm
and
88 cm
-1
; KZnF 3:25 cm
-1
,
unusual temperature dependence,
and possibly KCoF
3 :23
as shown in Figs. V-1,
cm
-1
-
) all
V-2,
; KMgF
show the
V-6,
3
:52
same
and V-7.
In
addition to broadening with increasing temperature these bands shift to higher frequencies, and this type of temperature dependence may be expected from, a very
QPR No.
89
(V.
OPTICAL AND INFRARED SPECTROSCOPY)
mm)
40
Fig. V-5.
60
80
100
120
140
WAVE NUMBER (cm - )
Transmission of K(Mgx:Ni 1 -x)F
40
60
80
100
at liquid-helium temperature
3
as a function of composition.
loosely bound impurity situated on a lattice site.
Here, the energy level scheme has
increasingly spaced levels that are populated when the temperature is raised and the
centroid of absorption moves to higher frequency.
Spin wave dispersion in antiferromagnetic KMnF
using neutron inelastic scattering. 4
at 4. 2 0 K has been observed when
From these results,
-1
q = 0. 8ql A-1 in the [100] direction was ~68 cm
(i. e.,
3
.
the magnon frequency at
No band in the region of 136 cm -1
two-magnon electric-dipole absorption) was observed and it is quite likely that
none of the bands in any of the antiferromagnetic perovskites can be attributed to twomagnon absorption (compared, for example, with those found 5 in MnF 2 ).
N. E. Tornberg has also investigated the Raman spectrum of several of the crystals
at
20°K,
but no
similar bands were found and there was only an indication of an
extremely weak two-phonon spectrum.
Again no bands were observed that could be
attributed to two-magnon transitions as observed in FeF 2 6
Most of the crystals investigated were subjected to spectrographic analysis to obtain
semiquantitative values for the impurity concentrations.
QPR No. 89
These are summarized in
az'
50
K(Zn: I%Ni) F
3
(1.7mm)
40-
SLIQ. He
---
LIQ. N2
30
20
--
I~-N
nvN
N
V
N
I
20
40
30
50
RbMnF 3
60
70
80
90
100
100
120
130
(1. mm)
401 --
LIQ. He
LIQ. N
2
az
40
Fig. V-6.
50
60
70
80
90
WAVE NUMBERS (cm
Transmission of K(Zn:l%Ni)F
liquid-nitrogen temperature.
3
)
and RbMnF
at liquid-helium and
3
U50
40
Fig. V-7.
o
z 30
3
at liquid-helium
and liquid-nitrogen temperature.
S20
20
Transmission of KCoF
30
40
WAVE
QPR No. 89
60
50
NUMBERS
70
cm 1
80
90
(V.
OPTICAL AND INFRARED SPECTROSCOPY)
Table V-2.
Impurity concentrations (in parts per million) for the
investigated fluoride crystals.
10 2-103
103-104
KNiF
3
#I
KNiF
3
#2
Ba, Sr
10-10
Na
Ca, Co, Al
-
Na
Ca, Mg
Ca
NaNi
KMgF
3
#1
-
KMgF
3
#557
-
Ca, NaNi
KMgF
3
#Tl
-
Ca, Na, Fe
KMgF
3
KMgF
3
#5
KZnF
3
(1% Ni)
RbMnF
KCoF
Na
O, Si,
-
3
Cl, Ca
Ca, Na
-
Ni
3
Fe
Fe
Ca, Mg,
Na
-
-
Ca, K,
-
-
Ca, Ni, Cu, Mg, Na, Zn
Na, Zn
Mass spectroscopic analysis.
Table V-2,
and fall approximately within the ranges specified.
In all samples Na and Ca are present,
responsible
for
the main
however, between KNiF
3
absorptions
and possibly these impurities are the ones
in
KMgF
3
and KNiF
3.
The main difference,
#1 and #2 samples (see Fig. V-3) is the large doping of Ba and
Sr in #1; these may be responsible for the additional lines seen in the KNiF
spectrum.
3
(#1)
There is some discrepancy between the mass spectroscopic analysis and the spectrographic analysis for the KMgF
cates,
however,
analysis),
3
the presence
sample.
The mass spectrographic analysis also indi-
of oxygen
and chlorine
(not detectable
by
emission
and no doubt all of the samples contain these substitutional impurities.
Consequently, although the spectra yield a large amount of information, it is virtually impossible, at this point, to make assignments and attempt theoretical calculations.
The materials obviously contain numerous impurities, and unless more pure starting
materials are available that are then doped with a known impurity, it appears to be a
hopeless task to identify any of the bands. Nevertheless, the results show that great
QPR No. 89
(V.
OPTICAL AND INFRARED SPECTROSCOPY)
precaution must be excercised in the interpretation of any weak band observed at low
-l
temperatures in this frequency range (10-150 cm- ) where absorptions attributable to
small concentrations of impurities can be considerably significant.
We would like to thank Dr. H. Guggenheim of the Bell Telephone Laboratories,
and Dr. A. Linz of the Materials Center, M. I. T.,
for many of the samples.
permitted comparisons between samples from different sources,
Inc.,
This
but even this additional
information has not aided in the interpretation.
D. J.
Muehlner, C.
H. Perry
References
1. C. H. Perry and E.
F.
Young, J.
Appl. Phys. 38, 4616 (1967).
2.
E. F.
H.
Perry, J.
Appl. Phys. 38, 4624 (1967).
3.
P.
4.
S. J.
Pickart,
5.
S. J.
Allen, Jr.,
Young and C.
J.
S. Richards,
Appl. Phys. 34,
M. F.
Collins, and C.
R. Loudon, and P.
1237 (1963).
G. Windsor, J.
L.
Richards,
Appl. Phys. 37, 1054 (1966).
Phys.
Rev.
Letters 16,
463
(1966).
6.
P. A. Fleury, S. P. S. Porto, L. E.
Rev. Letters 17, 84 (1966).
QPR No. 89
Cheeseman, and H. J.
Guggenheim,
Phys.